iommu/amd: Move and introduce new IRTE-related unions and structures

Move existing unions and structs for accessing/managing IRTE to a proper
header file. This is mainly to simplify variable declarations in subsequent
patches.

Besides, this patch also introduces new struct irte_ga for the new
128-bit IRTE format.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>